index with 0 rows rendered unusable

[oracle@stormking cdb12102 partition]$ sqlplus /nolog @ conn.u.sql

SQL*Plus: Release 12.1.0.2.0 Production on Sat May 21 12:40:34 2016

Copyright (c) 1982, 2014, Oracle.  All rights reserved.

Connected.
SQL> @ demo.not.unusable.sql
SQL> set linesize 200
SQL> set trimspool on
SQL> column table_name format a30
SQL> column index_name format a30
SQL> column partition_name format a30
SQL> set echo on

SQL> create table sherlock
  2  (
  3  season     number not null,
  4  episode number not null,
  5  release date not null,
  6  title      varchar2(40) not null,
  7  moriarty        varchar2(40) null,
  8  ireneadler varchar2(40) null
  9  )
 10  partition by range ( release )
 11  (
 12  partition p201008 values less than ( to_date( '201009','yyyymm' ) ),
 13  partition p201009 values less than ( to_date( '201010','yyyymm' ) )
 14  );

Table created.

SQL> create index sherlock_pk on sherlock ( season, episode, release ) local;

Index created.

SQL> alter table sherlock add constraint sherlock_pk primary key ( season, episode, release );

Table altered.

SQL> create index ireneadler_local_idx on sherlock ( ireneadler ) local;

Index created.

SQL> create index moriarty_global_idx on sherlock ( moriarty );

Index created.

SQL> insert into sherlock ( season, episode, release, title, moriarty, ireneadler )
  2  values ( 1, 1, to_date('20100710', 'yyyymmdd' ), 'A Study in Pink', null, null );

1 row created.

SQL> insert into sherlock ( season, episode, release, title, moriarty, ireneadler )
  2  values ( 1, 2, to_date('20100810', 'yyyymmdd' ), 'The Blind Banker', null, null );

1 row created.

SQL> commit;

Commit complete.

SQL> begin
  2          dbms_stats.gather_table_stats(
  3                  null,
  4                  'SHERLOCK'
  5          );
  6  end;
  7  /

PL/SQL procedure successfully completed.

SQL> @ global.index.status.sql
SQL> select i.table_name, i.index_name, i.num_rows, i.status
  2  from user_indexes i
  3  where i.table_name = 'SHERLOCK'
  4  and status != 'N/A'
  5  order by i.index_name;

TABLE_NAME                     INDEX_NAME                       NUM_ROWS STATUS
------------------------------ ------------------------------ ---------- --------
SHERLOCK                       MORIARTY_GLOBAL_IDX                     0 VALID

SQL> @ local.index.status.sql
SQL> select i.table_name, ip.index_name, ip.partition_name, ip.num_rows, ip.status
  2  from user_ind_partitions ip
  3  join user_indexes i on ip.index_name = i.index_name
  4  where i.table_name = 'SHERLOCK'
  5  order by ip.index_name, ip.partition_position;

TABLE_NAME                     INDEX_NAME                     PARTITION_NAME                   NUM_ROWS STATUS
------------------------------ ------------------------------ ------------------------------ ---------- --------
SHERLOCK                       IRENEADLER_LOCAL_IDX           P201008                                 0 USABLE
SHERLOCK                       IRENEADLER_LOCAL_IDX           P201009                                 0 USABLE
SHERLOCK                       SHERLOCK_PK                    P201008                                 2 USABLE
SHERLOCK                       SHERLOCK_PK                    P201009                                 0 USABLE

SQL> alter table sherlock split partition p201008
  2  at ( to_date( '201008','yyyymm' ) )
  3  into ( partition p201007, partition p201008 )
  4  ;

Table altered.

SQL> @ global.index.status.sql
SQL> select i.table_name, i.index_name, i.num_rows, i.status
  2  from user_indexes i
  3  where i.table_name = 'SHERLOCK'
  4  and status != 'N/A'
  5  order by i.index_name;

TABLE_NAME                     INDEX_NAME                       NUM_ROWS STATUS
------------------------------ ------------------------------ ---------- --------
SHERLOCK                       MORIARTY_GLOBAL_IDX                     0 UNUSABLE

SQL> @ local.index.status.sql
SQL> select i.table_name, ip.index_name, ip.partition_name, ip.num_rows, ip.status
  2  from user_ind_partitions ip
  3  join user_indexes i on ip.index_name = i.index_name
  4  where i.table_name = 'SHERLOCK'
  5  order by ip.index_name, ip.partition_position;

TABLE_NAME                     INDEX_NAME                     PARTITION_NAME                   NUM_ROWS STATUS
------------------------------ ------------------------------ ------------------------------ ---------- --------
SHERLOCK                       IRENEADLER_LOCAL_IDX           P201007                                   UNUSABLE
SHERLOCK                       IRENEADLER_LOCAL_IDX           P201008                                   UNUSABLE
SHERLOCK                       IRENEADLER_LOCAL_IDX           P201009                                 0 USABLE
SHERLOCK                       SHERLOCK_PK                    P201007                                   UNUSABLE
SHERLOCK                       SHERLOCK_PK                    P201008                                   UNUSABLE
SHERLOCK                       SHERLOCK_PK                    P201009                                 0 USABLE

6 rows selected.

SQL> alter index moriarty_global_idx rebuild;

Index altered.

SQL> alter table sherlock drop partition p201008;

Table altered.

SQL> @ global.index.status.sql
SQL> select i.table_name, i.index_name, i.num_rows, i.status
  2  from user_indexes i
  3  where i.table_name = 'SHERLOCK'
  4  and status != 'N/A'
  5  order by i.index_name;

TABLE_NAME                     INDEX_NAME                       NUM_ROWS STATUS
------------------------------ ------------------------------ ---------- --------
SHERLOCK                       MORIARTY_GLOBAL_IDX                     0 UNUSABLE